1. Field of the Invention
The present invention relates to a vessel for holding molten metal and supplying the same to a casting die after filtering impurities contained in the molten metal.
2. Description of Related Art
A vessel for holding molten metal therein and for filtering impurities contained in the molten metal is disclosed in JP-A-7-71880. Impurities such as metal oxides or other foreign particles are removed from the molten metal before the molten metal is supplied to a casting die. The vessel disclosed in the above-publication is briefly shown in FIG. 8 attached hereto.
The vessel 9 includes a melting chamber 97 and a reservoir chamber 98. A metal ingot is supplied to the vessel from an opening 99 and melted in the melting chamber. A burner 91 is submerged in the molten metal 2. An impurity separator 92 is also submerged in the molten metal and blows out inert gas therefrom to bring impurities contained in the molten metal on the surface of the molten metal. A ceramic filter 93b held by a hollow filter holder 93a is located at an inlet port of the reservoir chamber 98. Impurities contained in the molten metal 2, such as metal oxides and foreign particles, are filtered through the filter 93b, and clean molten metal is supplied to the reservoir chamber 98. An upper opening of the reservoir chamber 98 is covered by a cover plate 95 which is opened when the molten metal is taken out. When the cover 95 is opened, inert gas is blown from an inert gas supplier 96 to prevent the molten metal from being oxidized.
An outer periphery of the filter 93b is tightly fastened to a vessel body to prevent the molten metal from flowing through without being filtered. The top portion of the filter holder 93a is fixed to the vessel body. The filter 93b has to be replaced periodically to keep the filter meshes clean. In this filter structure, both the filter holder 93a and the filter 93b have to be replaced together, thereby making the maintenance cost high.
The filter structure may be modified by replacing the whole filter 93 with a filter plate, the outer periphery of which is fastened to a groove formed on the vessel body to prevent leakage flow of the molten metal and the top of which is sticking out above the molten metal surface. When the molten metal is aluminum, for example, the vessel body is made of a material obtained by sintering a mixture mainly composed of silicon oxide and alumina. Though the vessel body material of this kind is hard to react with the molten aluminum kept at a high temperature, it is difficult to keep the vessel body completely unaffected by the molten aluminum. Especially on the surface of the molten metal where the molten metal is easily oxidized, aluminum oxides stick to the wall of the vessel body. Once a spot of the aluminum oxides is formed on the wall, the aluminum oxides gradually grow on the wall around the spot. The filter periphery fastened to the groove formed on the vessel wall will be tightly fixed to the wall by the aluminum oxides, and the shape of the filter periphery will be deformed. As a result, it becomes difficult to replace the filter for maintenance, and moreover a new filter cannot be fastened to the groove.
Further, in the conventional vessel shown in FIG. 8, it is difficult to completely prevent oxidation of the molten metal surface when the cover 95 is open to take out the molten metal even inert gas is blown toward the surface. The molten metal is taken out from the vessel with a ladle. Therefore, the molten metal is oxidized by contacting air, and solid oxides are formed on the surface of the molten metal, which in turn are mixed into the molten metal. Thus, it is difficult, in the conventional vessel, to supply clean molten metal to the casting die.